The present invention relates to a multipass wiredrawing machine provided with a device for adjusting and controlling the tension of the wire being drawn, particularly suitable for drawing metal wires and the like.
In particular, the invention relates to multiple soap and wet wiredrawing machines.
The term "wiredrawing" refers to a form of plastic cold working consisting in the forced passage of the material through the shaped bore of a die, performed by pulling. The series arrangement of multiple dies with holes having progressively decreasing diameters allows to reduce the initial diameter to the selected size.
In order to draw metal wires the wire passes through a die, where its cross-section is reduced by plastic deformation.
Traction is applied by a traction drum which is located downstream of the die and, by rotating, winds up a certain number of turns of the wire.
In the case of multipass wiredrawing machines, this operation is repeated several times in succession, guiding the wire towards the subsequent stage; accordingly, multiple dies are provided and arranged mutually in series, and each die has a corresponding drum arranged downstream.
The wire is thus guided from one drum to the next die, with a consequent reduction in diameter between one pass and the next.
Traction must be applied because the wire being drawn, by causing friction against the traction drum, which is driven by an electric motor, allows said drum to pull it (with a capstan-like effect), wind it and then guide it on to the next drum.
Since wiredrawing occurs without removing material, i.e., with a constant volume, the ratio between the cross-section and the speed of the wire becomes particularly important and must be kept constant for each pass through the different dies and respective drums.
Accordingly, every time the wire diameter decreases, the wire becomes longer and therefore each drum has a progressively faster rotation rate in order to wind an increasingly longer wire.
The ideal situation would be one in which the wire passes directly from one drum to the next, without interposed wire tension control elements which, by acting directly on the wire, may damage it.
The basic requirement for correct operation is that the rotation rate of the individual drums must strictly match the tension changes of the wire as it leaves the respective dies, adapting to the elongations that the wire is subjected to as it passes through the various dies.
In practice, this theoretically ideal condition is difficult to achieve because it is difficult to precisely control the speeds of the individual motors that drive the drums and because of the variables that come into play during drawing, such as die wear etcetera.
In order to obviate these drawbacks, wiredrawing machines which accumulate wire between one drum and the next and, more recently, dancer-roll or bend-detector machines have been used.
These last devices are moving elements onto which the wire being drawn is guided. Any variations in the elongation of the wire produce angular or linear movements of these elements which, through position transducers, such as inductive transducers (potentiometers, encoders, etcetera) control and correct the speed of the corresponding motor, thus maintaining continuous drawing.
Conventional solutions described above, however, have the drawback that they bend the wire around the moving elements, causing bending which is sometimes excessive if obtaining optimum metallurgical characteristics for said wire is sought.
Moreover, drawbacks due to the constructive difficulties of wiredrawing machines using these solutions and difficulties in threading the machines are present.
Another solution that has been adopted recently to provide wiredrawing machines whose operation complies as much as possible with the above-described optimum condition entails controlling the speed of the individual drums by detecting wire tension changes both in the portion between the die and the drum and in the portion between the drum and the following die.
The position of the drums and of the dies is fixed; accordingly, since no moving parts are provided, control of the speed of the drum motors generates variable tensions on the wire in the portion between the drum and the next die.
Another problem that is encountered with wiredrawing machines relates to the fact that very often, due to malfunctions or to production requirements, it is necessary to exclude one or more passes of the wire through the die.
In current machines it is relatively easy to exclude a unit by acting on appropriately provided controls provided on the control console of the apparatus. In practical operation, the wire is guided onto the drum that follows the excluded drum, which remains motionless. In the case of a linear wiredrawing machine, without dancer rolls or bend detectors, one or more guiding rolls, fitted in a suitable position, are often used in order to prevent the wire from sliding against the excluded drum. Due to assembly and positioning requirements, the diameter of these rolls must be limited; accordingly, this fact entails negative aspects which affect the quality of the resulting product.
In fact, it has been observed that any contact of the wire with other elements that change its orientation produces a relative sliding which alters the layer of lubricant, i.e., stearate, on the surface of the wire, consequently varying its intrinsic characteristics.
The smaller the guiding roll or rolls used, the greater this problem becomes.